CMFActivity.Activity.SQLBase: Do not increment priority after failure

When an activity failure happens, the SQL row is updated:
- date is set to a future value, so the activity does not get retried
  immediately, in the expectation that what caused the failure may have
  cleared by that point.
- retry is incremented, to allow limiting the total number of retries
- priority is incremented

This last point seems harder to justify, and seems redundant with the date
increase. In the context of processing node families and with a steady
influx of similar activities at a base priority level, such priority
increment can postpone the victim activity execution to an arbitrarily
large amount of time, which is undesirable.
So, remove this increment.